I plan to use the two-microelectrode technique of voltage clamping the sheep cardiac Purkinje fiber to examine in detail how cardiac contraction depends on membrane voltage, ionic currents, and time. The principal goal of the project is to examine the behavior of the slow inward current (Isi) and its relation to contraction. Specific projects include: a re-examination of the ionic currents flowing during the plateau of the Purkinje fiber action potential; attempts to separate Isi from other currents so that the kinetics of Isi can be analyzed; analysis of the kinetics of Isi; comparison of Isi kinetics to those of contraction; and experiments designed to examine the factors governing the rate of recovery of the contractile system in heart.